1. Field of the Invention
The present invention relates to methods of producing hydrogen storage alloys used in such applications as hydrogen storage materials, hydrogen absorption materials for use in thermal conversion, hydrogen supply materials for use in fuel cells, negative electrode materials for use in Ni-hydrogen batteries, hydrogen refining and recovering materials, and hydrogen absorption materials for use in hydrogen gas actuators, in particular to methods of producing alloys excellent in their performance under environment temperatures (20 to 80° C.).
2. Related Art
Conventionally, there has been a compressed gas system or liquid hydrogen system as an instrument for storing and transporting hydrogen, however, in place of these methods, a method that uses hydrogen storage alloys has been gaining attention. As is generally known, the hydrogen storage alloys react reversibly with hydrogen and, accompanying the release and absorption of the heat of the reaction, absorb and release hydrogen. By making use of the chemical reaction, a method of storing and transporting hydrogen has been attempted for practical use, and furthermore, by making use of the heat of reaction, technology constituting a heat storage or heat transportation system has been attempted to be developed and put into practical use. As typical hydrogen storage alloys, such as LaNi5, TiFe, and TiMn1.5 are well known.
In putting various kinds of applications into practical use, the performance of hydrogen storage materials is necessary to be further improved. For instance, an increase in an amount of storable hydrogen, an improvement in the plateau property, and an improvement in the durability can be cited as main problems.
Metals having body-centered cubic structure (hereinafter referred to as a BCC structure) such as, for instance, V, TiVMn- and TiVCr-based alloys, have been long since known to be capable of absorbing more hydrogen than AB5 type alloys and AB2 type alloys that have been put into practical use, and are considered to be promising as the hydrogen storage alloys capable of use in the above various kinds of applications.
However, in the hydrogen storage alloys having the BCC structure, an amount of hydrogen that can be effectively absorbed and released is substantially only a half of that based on theory. That is, these alloys are not sufficient in practical use as storage materials.
That is, these kinds of hydrogen storage alloys, including the hydrogen storage alloys having the BCC structure, are mainly produced according to a melting method. According to the method, the homogenization is carried out by applying heat treatment and rapid solidification, however, because of the presence of more or less segregation, precipitates or inclusions, complete homogenization cannot be attained. As a result, there is a drawback in that according to a ratio of the segregation, there is observed deterioration in the plateau property or a decrease in the rechargeable hydrogen capacity. Furthermore, there is a practical problem in that since repetition of the absorption and release of hydrogen greatly damages the alloy, as the number of the repetition cycles increases, an equilibrium dissociation pressure largely decreases. In particular, there is tendency that in the hydrogen storage alloys having the BCC structure the drawbacks are remarkable.